A fireball that dazzled Americans on July 23rd was
a piece of a comet or an asteroid, scientists say. Contrary to
reports, however, it probably didn't scorch any cornfields.

July
27, 2001: Every few weeks, somewhere on Earth, a fiery light
streaks across the sky casting strange shadows and unleashing
sonic booms. Astronomers call them fireballs or "bolides."
They're unusually bright meteors caused by small asteroids that
disintegrate in our planet's atmosphere. Often they explode high
in the air like kilotons of TNT -- blasting tiny meteorites far
and wide.

It happens all the time, say experts, but usually no one notices.
We live on a big planet, after all, and very little of Earth's
surface is inhabited by people. Most debris from space falls
unseen over oceans or sparsely-populated land areas -- or during
times when sky watchers simply aren't paying attention.

Above: Artist Duane
Hilton created this rendition of the July 23rd fireball streaking
over a Pennsylvania farmhouse.

Last Monday was different, however. On July 23rd hundreds
of thousands of people were looking when, unexpected, a fireball
appeared over the US east coast. It was 6:15 p.m. local time.
The Sun hadn't set, but onlookers had no trouble seeing the fireball
in broad daylight. Witnesses from Canada to Virginia agreed that
the colorful fireball was brighter than a Full Moon, and some
saw a smoky trail lingering long after it had passed.

"Contrary to some reports this was not a meteor
shower," says Donald Yeomans, manager of NASA's Near Earth
Object program at the Jet Propulsion Laboratory. Meteor showers
happen when Earth passes through the debris trails of comets
and countless thousands of cosmic dust specks burn up in Earth's
atmosphere. At the heart of Monday's fireball, however, was a
solitary object -- perhaps a small asteroid or a piece of a comet.

Hundreds of eyewitness reports collected by the American Meteor
Society establish that the fireball was moving on an east-west
trajectory that carried it directly over the state of Pennsylvania.
"It was traveling perhaps 15 km/s (34,000 mph) or faster
when it exploded in the atmosphere with the force of about 3
kilotons of TNT," says Bill Cooke, a member of the Space
Environments team at the Marshall Space Flight Center. If this
was a rocky asteroid, then it probably measured between 1 and
2 meters across and weighed 30 or so metric tons.

"The pressure wave from the airburst shattered some windows
in towns west of Williamsport," Cooke continued. "Breaking
glass requires an overpressure of about 5 millibars (0.5 kPa),
which means that those homes were within 100 km of the explosion."

No
one knows if any sizable fragments of the object survived the
blast. But if they did, the meteorites probably landed in the
wooded, hilly terrain west of Williamsport -- perhaps in one
of the many state parks of that area.

Left: Jim Richardson of the American Meteor Society
created this July 23, 2001, fireball sighting map. Red stars
denote witness locations; the tail on each star points in the
direction that the fireball was spotted. Blue stars denote sonic
booms. The green rectangle and arrows indicate the approximate
trajectory of the fireball. [more]

Says Bob Young of the State Museum of Pennsylvania: "One
of our planetarium staff was told that the little northern Pennsylvania
town of Trout Run was destroyed by the meteor! The witness was
about 100 miles away when she heard the tale from her hairdresser."
Other reports credit the fireball for scorching
a cornfield in Lycoming County, PA, and littering the countryside
with burnt rocks.

In fact, says Yeomans, it's unlikely that any substantial
meteorites reached the ground. Atmospheric friction would have
reduced most of the fragments to dust. Even if fragments did
survive, he added, they wouldn't burn cornfields because --despite
their fiery appearance in the sky-- freshly-fallen meteorites
are not hot.

Objects from space that enter Earth's atmosphere are -- like
space itself -- very cold and they remain so even as they blaze
a hot-looking trail toward the ground. "The outer layers
are warmed by atmospheric friction, and little bits flake away
as they descend," explains Yeomans. This is called ablation
and it's a wonderful way to remove heat. (Some commercial heat
shields use ablation to keep spacecraft cool when they re-enter
Earth's atmosphere.) "Rocky asteroids are poor conductors
of heat," Yeomans continued. "Their central regions
remain cool even as the hot outer layers are ablated away."

Right:
This ablative
heat shield protected a space probe in 1986 as it made a
high-speed plunge from NASA's Galileo spacecraft into the atmosphere
of Jupiter. Meteorites racing through Earth's atmosphere likewise
shed heat via ablation. [more
information]

Asteroids move faster than the speed of sound in Earth's atmosphere.
As a result, the air pressure ahead of a fireball can substantially
exceed the air pressure behind it. "The difference can
be so great that it actually crushes the object," says Cooke.
"This is probably what triggered the airburst over Pennsylvania."

Small fragments from such explosions lose much of their kinetic
energy as they heat the atmosphere via friction. They quickly
decelerate and become sub-sonic. Dusty debris from airbursts
(and ablation) can linger in the atmosphere for weeks or months,
carried around the globe by winds. Walnut- to baseball-sized
fragments might hit the ground right away at a few hundred kilometers
per hour.

"Small rocky meteorites found immediately after landing
will not be hot to the touch," says Yeomans. They will not
scorch the ground or start fires. On the other hand, notes Cooke,
"if we got hit by something large enough to leave a crater,
the fragments might be very hot indeed." A stony meteorite
larger than 50 meters might be able to punch through the atmosphere
and do such damage -- but that's far larger than the object that
flew over Pennsylvania.

No one knows what kind of space debris caused the July 23rd
fireball. It might have been a small piece of an icy comet, in
which case it's unlikely that anything larger than dust grains
survived. It might also have been a rocky asteroid -- the most
likely candidate -- or perhaps a nickel-iron meteorite. "Iron
objects are more likely to survive a descent to Earth,"
says Yeomans, "but they are rare."

It's
possible that fragments will never be found, notes Cooke. "We
still don't have a precise trajectory for this object,"
he explains. "And so much of the targeted area (in central
Pennsylvania) is heavily forested -- searching for debris will
be like looking for a needle in a haystack."

Or should that be a needle in a cornfield?

"I suppose it's possible that some ablative fragments
fell into that field," says Cooke, "but it is strange
that only a small area was affected. I doubt it's a good candidate
impact site."

Editor's note: Did you see the July 23rd fireball?
If so, please submit
a report to the American Meteor Society. They can use your
information to refine the trajectory of the meteor and possibly
pinpoint the location of meteoritic debris. Also, the terms fireball
and bolide are often confused -- even by professional astronomers.
A fireball is a meteor at least as bright as the planet Venus
(visual magnitude -3 or -4). A bolide is a fireball that explodes,
often with sound effects.

Speeding in Space: Scientists say the Pennsylvania fireball
ripped through the atmosphere at 15 km/s. In this math activity,
students will compare the fireball's speed to the speed of Earth's
orbital motion around the Sun -- and practice their English to
metric conversions!
[lesson
plan] [activity
sheet] [key]

Meteorite leaves trail of fire,
confusion
-- (CNN) A streaking fireball or fireballs witnessed over much
of the eastern United States seems to have disappeared without
a trace, save perhaps for strange markings in a Pennsylvania
cornfield.

Arctic
Asteroid!
-- Science@NASA A 200 metric ton rock from
space streaked across the skies of western Canada on January
18, 2000 and scattered intriguing meteorites across a frozen
lake.

: In 1992 a fireball raced over the eastern US
and dumped a 12-kg rocky meteorite in the trunk of a parked car
in Peekskill, NY. At least 14 people captured videos of the meteor
-- one of which is reproduced here as a 0.9
MB mpeg movie. [more]